Physiology of the Respiratory System – Partial Pressures of Respiratory Gases

Vocabulary flashcards covering definitions and key facts about partial pressures, gas composition, ventilation effects, and the oxygen cascade in the respiratory system.

Partial pressure

The pressure exerted by an individual gas within a mixture of gases.

Dalton’s law of partial pressures

The total pressure of a gas mixture equals the sum of the partial pressures of each constituent gas (Ptotal = PA + PB + PC …).

Partial-pressure equation

Partial pressure of a gas = Total (barometric) pressure × Fractional concentration of the gas.

Factors affecting partial pressure

Barometric (total) pressure and the fractional concentration of the gas.

Fractional concentration of O₂ in dry air

≈20.98 % of total dry atmospheric gas content.

Fractional concentration of N₂ in dry air

≈78.06 % of total dry atmospheric gas content.

Fractional concentration of CO₂ in dry air

≈0.04 % of total dry atmospheric gas content.

Barometric pressure at sea level

760 mm Hg (1 atmosphere).

Partial pressure of O₂ in dry air at sea level

≈159 mm Hg (760 mm Hg × 0.2098).

Functional residual capacity (FRC)

≈2 L of air remaining in the lungs after a quiet expiration; helps stabilize alveolar gas composition.

Tidal volume (TV)

≈500 mL of air moved in or out during a normal quiet breath; only ~350 mL reaches the alveoli.

Alveolar gas stability

Large FRC compared with small tidal volume keeps PAO₂ and PACO₂ nearly constant at rest.

Effect of increased ventilation

Raises PAO₂, lowers PACO₂, and makes alveolar gas composition approach inspired air composition.

Oxygen cascade

Stepwise fall in PO₂ from inspired air (≈158 mm Hg) → alveoli (≈100 mm Hg) → arterial blood (≈95 mm Hg) → tissues (≈10 mm Hg in mitochondria) → mixed venous blood (≈40 mm Hg).

Inspired PO₂ (PiO₂) at sea level

≈158 mm Hg after humidification of ambient air.

Alveolar PO₂ (PAO₂)

≈100 mm Hg under normal resting conditions.

Arterial PO₂ (PaO₂)

≈95 mm Hg in systemic arterial blood due to physiological shunt and V/Q matching.

Mixed venous PO₂ (PvO₂)

≈40 mm Hg in blood returning to the lungs after tissue oxygen extraction.

Mitochondrial PO₂

≈10 mm Hg, reflecting the low intracellular oxygen tension where oxidative metabolism occurs.

Physiological shunt

Small portion of venous blood bypasses ventilated alveoli, lowering PaO₂ relative to PAO₂.

Pressurised aircraft cabin altitude

Cabins are maintained at ≈6000–8000 ft (≈565 mm Hg), equivalent to breathing ~15 % O₂.

Cabin PiO₂

≈118 mm Hg, causing SaO₂ to drop to ~90 % in healthy passengers and lower in patients with lung disease.